This page introduces you
to the principles of electrolysis and explains all the technical terms
you need to know about e.g. ions, electrolyte, non–electrolyte, cell,
electrodes, electrode equations, negative cathode electrode, positive
anode electrode. All the technical terms are defined and explained. The
apparatus you can use are described and seven examples of electrolysis
processes are described in detail (links 2. to 7. in the index at the
end of each page). These revision notes on an introduction to
electrolysis should prove useful for the new AQA chemistry, Edexcel
chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

Electrolysis is the process of
electrically inducing chemical changes in a conducting melt or solution e.g.
splitting an ionic compound into the metal and non–metal or producing gases like
hydrogen, oxygen and chlorine from salt solutions.

SUMMARY OF COMMON ELECTRICAL CONDUCTORS

What makes up a circuit in cells –
batteries or electrolysis? What carries the current?

Conductors are materials (solid or
liquid) that carry an electric current
via freely moving electrically charged particles, when a potential
difference (voltage!) is applied across them, and they include:

All metals (molten or solid),
non–metal carbon (graphite) and some recently developed 'smart
materials' (which get no further mention in this section of notes).

This conduction involves the movement of
free or delocalised electrons (e– charged particles) and does
not involve any chemical change.

That doesn't mean to say these
'electrical currents' can't promote chemical change, which happens as we
will see later in the process we call electrolysis.

Any compound, molten or dissolved in
solution, in which
the liquid contains free moving ions is called the
electrolyte and can conduct an electrical current. (see
non–electrolyte)

Ions are electrically charged particles e.g.
Na+sodium ion, or Cl– chloride ion, and their movement or flow
constitutes an electric current, in other words the electrolyte is an
electrically solution containing mobile ions that conducts electricity as a
stream of moving charged particles.

An electrolyte may consist of ...

(i) a molten ion compound i.e. on
melting the ions are free to move to carry the current, or

(ii) any compound that dissolves in a
liquid to give a solution of ions that are free to move.

The compound is usually ionic and the
liquid is usually water, so in most of the examples described, the
electrolyte is an aqueous solution of ions with a few molten salt
examples).

Note: Aqueous means to do with water, so
an aqueous solution is a solution made by dissolving something in
the solvent water.

When an electric current is passed through
such an electrolyte chemical changes occur on the electrical contacts
(called electrodes) and chemical changes happen to break down the
compound in a process called electrolysis.

Water is very poor conductor because it
is a covalent compound and only minute amounts of it ionises to form
hydrogen and hydroxide ions, so water it is not an effective
electrolyte.

The majority of liquid water
consists of covalent H2O molecules, but there
are trace quantities of H+ and OH– ions from the
self–ionisation of water,

H2O(l)
H+(aq) + OH–(aq)

(about 1 in 200 million does
this!, the reaction is reversible, so the longer half-arrow to the left
tells you that most water remains as water molecules!).

However, once you
dissolve an ionic salt like compound or a stronger acid, water (as
an aqueous solution) becomes a good electrical conductor i.e. a good
electrolyte.

Apart from metals, most solids do not conduct
electricity because there are no free electrons to carry an electric
current.

However, although molten ionic compounds and
solutions in of ionic compounds in water can carry a current (and undergo
electrolysis - chemical change) solid ionic compounds cannot conduct
electricity and undergo electrolysis. This is because the ions are tightly
held in the crystal lattice and cannot move around and migrate to any
electrical contact placed on the solid.

When an appropriate d.c. current is
passed through an electrolyte, chemical changes occur where the external
circuit connections (electrodes) are dipped into the electrolyte.

These chemical changes ONLY occur on the surface of the electrodes where
they are in contact with the electrolyte solution and (usually) elements are released as the compound is
broken down by the process called electrolysis.

What does the complete electrical
circuit for electrolysis consist of?

The
electrolyte container can be made from a short piece of wide glass
tubing with a rubber bung base with two holes drilled in it to take to
carbon rods as electrodes. Small test tubes filled with electrolyte are
inverted over the carbon electrodes.

This simple set-up is recommended by the RSC and consists of two wire
electrodes bent in a S shape so the gases can be collected in little
test tubes filled with the electrolyte and inverted over the electrodes
in the beaker of electrolyte.

The diagrams above illustrate simple
electrolysis experiments you will see or (hopefully) do in a school
laboratory or
college laboratory.

The electrolyte solution (in this
case sulfuric acid, can be sodium chloride etc.) is contained within the
electrolysis cell (e.g. section of wide plastic piping).

Two electrical connectors called electrodes protrude upwards into the solution (e.g. graphite/carbon
rods} pushed through two holes drilled in a larger rubber bung. This is
the same function as the two wires in the other electrolysis set-up
illustrated above.

The circuit is completed when connected
to an external electrical power supply of d.c. current, and usually a
voltage of 2-3 V is quite sufficient to give a good rate of electrolysis.

So, in sequence from the negative
terminal, through the external copper wire electrons flow clockwise from
the positive electrode to
the negative electrode (cathode), then ions (NOT
electrons) carry the current through the electrolyte across to the
positive electrode (anode), and then
electrons again carry the current through another external wire to the
positive terminal of the battery or other power supply.

When you switch on the d.c. power on, or
connect the battery, the electrolysis process should start. Often,
but not always, you will see bubbles of gas appearing on the electrode surface, because that's where the
chemical changes
we call electrolysis take place!

Flowing in one direction only, the
electrons carry the current in the external copper wires BUT not in the
electrolyte solution. However in the electrolyte solution there are two ion currents flowing in opposite directions,
and its important that this is understood because no chemical change can
take place if the ions are not attracted to their oppositely charged
electrode.

All salts and many acids are good
electrolytes, good electrical conductors of a d.c. current because
they provide high concentrations of positive and negative ions.

It is possible to
demonstrate this flow using a coloured
ion experiment (see diagram and text below).

Remember no electrons flow in
the solution, but they do flow in metal wires or carbon
(graphite) electrodes of the external circuit.

When an ion meets its oppositely
charged electrode, one of two things can happen. Either the ion
hangs around the electrode and does nothing OR the ion undergoes
chemical change, sometimes referred to as 'the ion is discharged'.

The chemical changes that occur
on the surface of an electrode are either a REDUCTION (on
the negative cathode electrode) or an OXIDATION (on the
positive anode electrode).

Each of the oxidation or
reduction changes is written as a half-equation, so you
see the electrons lost or gained

At the negative cathode
electrode, positive ions (cations) are attracted and these
positive ions may gain electrons and are reduced to
some chemical product e.g. typical half-reactions ...

Either hydrogen gas or a metal
deposit is formed on the negative cathode electrode.

(ii) hydrogen and metals
are formed at the negative cathode electrode,

(iii) not all the
positive ions will be discharged i.e. reduced, so in a
mixture of H+ and Na+ ions in aqueous
solution, the hydrogen ions are preferentially reduced to
hydrogen, leaving the sodium ions unchanged in solution,

AND generally speaking, the
less reactive a metal, the more easily its ion is reduced to
the metal on the electrode surface e.g. in a mixture of
positive ions the preference order is

Cu2+ (==>
Cu) > H+ (==> H2) > Na+
(==> Na)

A general rule with reference to the
reactivity series of metals:

If the metal in the salt solution
is more reactive than hydrogen, then hydrogen the
hydrogen ion is most likely to be discharged at the negative cathode giving
hydrogen.

If a reactive metal like sodium was
discharged, it would immediately react with water giving hydrogen
anyway!

If the metal in the salt solution
is less reactive than hydrogen it is the metal ion that
is likely to be discharged forming a deposit of the metal
on the electrode surface.

In practice you can get a deposit of lead
from a lead nitrate solution.

At the positive anode
electrode, negative ions (anions) are attracted and these
negative ions may lose electrons and are oxidised
to some chemical product e.g. typical half-reactions ...

A non-metal like oxygen or
chlorine is released at the positive anode electrode.

(ii) apart from hydrogen, other non–metals are formed at the positive anode electrode,

(iii) AND not all the
negative ions will be discharged i.e. oxidised, so in a
mixture of OH– and Cl– ions in aqueous
solution, the chloride ions are preferentially oxidised to
chlorine, leaving most of the hydroxide ions unchanged in solution.

Oxygen is usually produced unless a halide
ion is present,

so chloride would give chlorine, bromide
gives bromine and iodide gives iodine at the positive anode.

All the above electrode
equations showing the formation of the electrolysis products
are fully described in the context of a laboratory experiment or
industrial process.

Three 'connected' sub–notes on
electrolysis investigations:
As well as investigating the products of electrolysis, you can also
vary experimental conditions e.g. changes in voltage p.d. or
electrolyte concentration can be studied.

(i) The greater the concentration of the electrolyte
ions, the lower the electrical resistance of the solution.
This is because there are more ions present to carry the current
e.g. if the voltage (V, volts) is kept constant, the
current flowing (I, amps) will steadily increase as the
concentration of the electrolyte is increased.

(ii) If the
electrolyte (ion) concentration is kept constant, the current
will steadily increase with increase in voltage
just like any other electrical circuit because the increase
in electrical field effect from the increased p.d. (voltage)
will force the ion flow at a greater rate.

The greater the voltage, the
faster the rate of electrolysis, but don't over do it!

The molten or dissolved materials are
usually acids, alkalis or salts and their electrical conduction
is usually accompanied by chemical changes
e.g. decomposition.

The chemical changes occur at the
electrodes which connect the electrolyte liquid containing ions with the
external d.c. electrical supply.

If the current is switched off, the
electrolysis process stops.

Non–electrolytes
are liquids or solutions that do not contain ions, do not conduct
electricity readily and cannot undergo the process of electrolysis
e.g. ethanol (alcohol), sugar solution etc. and are usually covalent
molecule liquids or solutions of covalent compounds. Even if a covalent
compound dissolves in water, if no ions are formed, there will be no
electrical conduction.

How do we know that ions move to the
electrodes when a d.c. current is applied?

The coloured ion experiment described below
illustrates how you can show the slow movement of ions when a voltage is
applies across a conducting solution of ions – the electrolyte.

A
rectangle of filter paper is soaked in an ammonia–ammonium chloride
solution and mounted on a microscope slide. The paper is connected to a
d.c. supply with clips. A 'line' of copper chromate solution is placed
in the middle of the paper and the current switched on. The
copper chromate is
green–brown in solution but gradually it disappears and
separates, in different directions, into a yellow and blue bands.
The yellow band is due to negative chromate ions, CrO42––,
moving towards the positive electrode. The blue
band is due to positive copper ions, Cu2+, moving towards the
negative electrode. All due to opposite charges attracting
in the electric field produced by the potential difference (the
voltage!).

In the
electrolyte, as with any solution, ALL the particles are moving around
at random, but they are not distributed at random as the electrolysis
proceeds.

During
electrolysis positive ions on average move towards the negative cathode
electrode,

AND the balancing
negative ions on average move towards the positive anode
electrode.

Both
streams of moving ions constitute the electric current flowing in the
electrolyte.

Two
examples are illustrated below, but the full electrolysis description
and explanation is given on individual web pages - see Electrochemistry
INDEX.

Diagram showing the net direction of ion movement in water acidified with
sulfuric acid.

Liquids that conduct must contain freely
moving ions to carry the current and complete the circuit.

You can't do electrolysis with an ionic
solid!, the ions are too tightly held by chemical bonds and can't
flow from their ordered situation! The particle theory of solids still
applies even if you try to pass electricity through it (apart from
graphite and metals).

When an ionically bonded substances are
melted or dissolved in water the ions are free to move about.

However some covalent substances
dissolve in water and form ions.

e.g. hydrogen chloride HCl, dissolves
in water to form 'ionic' hydrochloric acid H+Cl–(aq)

The solution of ions (e.g. salts, acids etc.)
or melt of ions (e.g. chlorides, oxides etc.) is called the
electrolyte which forms part of the circuit. The circuit is
completed by e.g. the external copper wiring and the (usually) inert
electrodes like graphite (form of carbon) or platinum AND electrolysis can
only happen when the current is switched on and the circuit complete.

and negatively charged ions move to the
positive electrode (anions attracted to anode), e.g. in the diagram,
chloride ions Cl–, move to the positive electrode (+ve).

The diagram shows the industrial electrolysis
process (in a Down's Process Cell) to extract sodium metal from sodium
chloride (common salt). This is an example of how electrolysis is used in
the chemical industry.

During electrolysis, gases may be given off,
or metals dissolve or are deposited at the electrodes.

Metals and hydrogen are formed at the
negative electrode from positive ions by electron gain (reduction), e.g.
in molten sodium chloride

sodium ions change to silvery grey
liquid sodium

Na+ + e–
==>
Na (a reduction electrode reaction)

and non–metals e.g. oxygen, chlorine,
bromine etc. are formed from negative ions changing on the positive
electrode by electron loss (oxidation), e.g. in molten sodium chloride

NaCl(aq) or
Na+ + Cl– for two possible
expressions of the aqueous solution

When dealing with the electrolysis of
aqueous solutions of salts and acids in water, things can be more
complicated and sometimes several competing electrode reactions can occur at
the same time, and sometimes products differ depending on the nature of the
cathode and anode electrodes.

There are links below (2. to 7.) which describe
in great detail particular examples of the process of electrolysis and all
can be done as college or school student experiments or teacher
demonstrations, your pupils can have great fun with these experiments but
take great care with the production of chlorine!